The present invention relates generally to a system and method for determining, with respect to a reference point, a depth of penetration of a leading edge of a rotating drill bit in a bore when the leading edge of the drill bit passes from a first medium having a first density to a second medium having a second density. More specifically, the present invention relates to a system and method for determining the length of either a unicortical or bicortical bore without removing the drill bit from the bore.
Inadequate and inaccurate depth measurement following orthopedic drilling procedures results in incorrect screw lengths which can lead to surgical complications. Furthermore, determining the correct screw length can be a time consuming procedure which is undesirable when tissue is exposed and subject to infection.
As shown in FIGS. 1A and 1B, the bony structure of the human anatomy consists mainly of cortical bone 10 having a hard outer cortex 12 and a soft inner medullary layer 14. Following traumatic injury, plate and screw placement is critical for adequate repair. Improper drilling lengths could lead to device instability, damage to anatomic structures, or device failure.
As shown in FIG. 1A , when using a rotating drill bit 16 to form a bicortical bore 18 through the cortical bone 10, the rotating drill bit 16 passes through a first portion 12a of the hard outer cortex 12, a soft non-resistant medullary layer 14, and a second portion 12b of the hard outer cortex 12.
As shown in FIG. 1B, when using a rotating drill bit 16 to form a unicortical bore 20 through the cortical bone 10, the rotating drill bit 16 passes through an entry point 22a of the hard outer cortex 12 and an exit point 22b of the hard outer cortex 12 without penetrating the soft non-resistant medullary layer 14.
Current techniques for drilling and screw placement are two-step processes at best. During an operation, a bore is first drilled by a surgeon until the surgeon xe2x80x9cfeelsxe2x80x9d the drill bit pass completely through the bony structure. A depth gage (not shown) is then inserted into the bore, grasped against the proximal end and a depth is recorded. A possible resulting complication of this procedure is that the surgeon may not precisely xe2x80x9cfeelxe2x80x9d the drill bit pass through the second cortical layer, thereby possibly damaging tissue on the opposite side of the bone. Another complication may occur if the depth gage is not properly inserted into the hole. If the gage is grasped prior to passing the distal end of the bore, a size will be determined that is smaller than the true depth.
The process of drilling and depth measurement often requires more than one attempt. Conservative drilling may result in incomplete drilling requiring multiple passes. Furthermore, multiple depth measurements may be obtained to confirm accurate placement of the gage. This process consumes a substantial amount of surgical time resulting in a large cost per patient. By combining the drilling and depth measurement process into one accurate procedure, cost is reduced along with a decrease in patient morbidity. What is needed is a simpler, cheaper, safer, faster and more accurate method and apparatus for measuring the depth of drill bit penetration and determining spatial location in a material having a varying density, such as cortical bone.
One aspect of the present invention is a drill bit penetration measurement system for determining, with respect to a reference point, a depth of penetration of a leading edge of a rotating drill bit in a bore when the leading edge of the drill bit passes from a first medium to a second medium. The first medium is contiguous with the second medium. The first medium has a first density. The second medium has a second density. The system comprising a first sensor, a second sensor, and a processor. The first sensor outputs a first signal representative of a displacement, with respect to the reference point, of the leading edge of the drill bit in the bore. The second sensor outputs a second signal representative of a force applied to the leading edge of the drill bit. The processor is in electrical communication with the first and second sensors. The processor is configured in a first mode to output a third signal representative of the depth of penetration of the leading edge of the drill bit when the leading edge of the drill bit passes from the first medium to the second medium. The third signal is based on the first and second signals.
Another aspect of the present invention is a drill bit penetration measurement system for determining, with respect to a reference point, a depth of penetration of a leading edge of a rotating drill bit in a bore when the leading edge of the drill bit passes from a first medium to a second medium. The first medium has a first density. The second medium has a second density. The drill bit is rotatably driven by a drive within a drill housing. The system comprises a drill bit displacement measurement assembly, a drill bit load measurement assembly and a controller assembly. The drill bit displacement measurement assembly is connected to the drill housing. The displacement measurement assembly has a first sensor outputting a first signal representative of a displacement of the leading edge of the drill bit in the bore. The drill bit load measurement assembly comprises a housing, a thrust assembly about which the housing is rotatable, a drill chuck and a second sensor. The housing has an axis of rotation. The housing is removably connected to the drive for rotation thereby. The drill chuck is connected to the housing for rotation therewith and to the thrust assembly for rotation with respect thereto. The drill chuck is movable in translation along the axis of rotation of the housing. The second sensor is connected to the thrust assembly. The second sensor outputs a second signal representative of a force applied to the drill bit. The control assembly is in electrical communication with the first sensor and the second sensor. The control assembly has a processor configured in a first mode to output a third signal representative of the depth of penetration of the leading edge of the drill bit when the leading edge of the drill bit passes from the first medium to the second medium. The third signal is based on the first and second signals.
Still another aspect of the present invention is a method for determining, with respect to a reference point, a depth of penetration of a leading edge of a rotating drill bit in a bore when the leading edge of the drill bit transitions from a first medium to a second medium. The first medium is adjacent the second medium. The first medium has a first density. The second medium has a second density. The method comprises: establishing as the reference point an initial position of the leading edge of the drill bit; outputting a first signal representing the depth of penetration of the leading edge of the rotating drill bit in the bore; outputting a second signal representing a force applied to the leading edge of the drill bit; and outputting a third signal representative of the depth of penetration of the leading edge of the drill bit when the leading edge of the drill bit passes from the first medium to the second medium. The third signal is based on the first and second signals.